1. Field of the Invention
The present invention relates to a substrate plasma processing apparatus of what is called a parallel plate type and a substrate plasma processing method, in which an RF electrode and a counter electrode are arranged facing each other in a vacuum chamber and a substrate held on the RF electrode is processed with plasma generated between the electrodes.
2. Description of the Related Art
When wiring or the like is performed on a substrate such as a semiconductor wafer, it is necessary to perform minute processing on the substrate. For this purpose, conventionally a plasma processing apparatus using plasma has been used frequently.
FIG. 11 is a diagram schematically showing the structure of an example of such a conventional substrate plasma processing apparatus.
The substrate plasma processing apparatus 10 shown in FIG. 11 is a plasma processing apparatus of what is called a parallel plate type. In the substrate plasma processing apparatus 10, a high frequency (RF) electrode 12 and a counter electrode 13 are arranged facing each other in a chamber 11. On a main surface of the RF electrode 12 that faces the counter electrode 13, a substrate S to be subjected to processing is held. A gas to be used for generating plasma and thereby for processing the substrate S is introduced from a gas introducing pipe 14 into the chamber 11 as shown by arrows. Along with this, a not-shown vacuum pump is used to evacuate the inside of the chamber 11 from an exhaust port 15. At this time, the pressure inside the chamber 11 is approximately 1 Pa for example.
Next, an RF (voltage) is applied to the RF electrode 12 via a matching device 16 from a commercial RF power supply 17 of 13.56 MHz. Thus, plasma is generated between the RF electrode 12 and the counter electrode 13.
At this time, positive ions in the plasma P are incident at high speed on the substrate S on the RF electrode 12 by a negative self-bias potential Vdc generated on the RF electrode 12. Consequently, the substrate incident energy at this time is used to induce surface reaction on the substrate S to thereby perform plasma substrate processing such as reactive ion etching (RIE), plasma chemical vapor deposition (PCVD), sputtering, ion implantation, or the like. Particularly, from a viewpoint of processing a substrate, RIE is mainly used. Therefore, the explanation below will be given mainly focusing on substrate processing using RIE in particular.
In the plasma processing apparatus as shown in FIG. 11, Vdc (average substrate incident energy) increases as the RF power increases as shown in FIG. 12. Accordingly, adjustment of the Vdc mainly by RF power is performed for adjusting a processing rate and adjusting a processing shape. Further, the Vdc can be partially adjusted also by the pressure or electrode shape on which the Vdc depends.
FIG. 13 shows a result of simulating parallel plate type Ar plasma with a frequency of 13 MHz, Vrf=160 V, pressure of 6.6 Pa, 30 mm distance between electrodes and 300 mm wafer size by a continuum model plasma simulator (G. Chen, L. L. Raja, J. Appl. Phys. 96, 6073 (2004)) to obtain an ion energy distribution. Further, FIG. 14 is a graph showing a distribution status of ion energy that is suitable for RIE of the substrate S.
The incident energy onto the substrate S exhibits an ion energy distribution as shown in FIG. 13. As is clear from FIG. 13, the ion energy in the plasma generated in the apparatus as shown in FIG. 11 is divided in two, a low energy side peak and a high energy side peak, and an energy width ΔE thereof becomes as wide as a few tens to a few hundreds eV depending on the plasma generating condition. Therefore, even when Vdc is adjusted to energy that is optimum for substrate processing, there exist ions having energy that is too high (high energy side peak) and ions having energy that is too low (low energy side peak) among ions incident on the substrate as shown in FIG. 14.
Therefore, in RIE for example, when substrate processing is implemented with ions having energy equivalent to the high energy side peak, there is a tendency to cause shoulder cutting (shoulder dropping) and deteriorate the processing shape. On the other hand, when substrate processing is implemented with ions having energy equivalent to the low energy side peak, it is equal to or lower than a surface reaction threshold and contributes nothing to the substrate processing or tends to deteriorate the processing shape accompanying deterioration of anisotropy (the ion incident angle widens by thermal velocity).
In such a viewpoint, in semiconductor processes in these days, it is necessary to narrow the band of the ion energy (realization of small ΔE) as shown by hatching at a substantially center portion in FIG. 14 and to optimally adjust an average energy value (optimization of Vdc) so as to finely control the processing shape corresponding to RIE of semiconductor devices, various films, and composite films, which are shrinking more and more.
To narrow the band of the ion energy, use of higher RF frequencies (for example, refer to JP-A 2003-234331 (KOKAI)) and use of pulse plasma (for example, refer to J. Appl. Phys. Vol. 88, No. 2, 643 (2000)) are considered.
Further, the plasma generation is roughly classified into an inductive coupling type and a capacitive coupling type. From a viewpoint of fine control of processing shape, it is effective to shorten a residence time by reducing the plasma volume so as to suppress secondary reaction. In such a viewpoint, the parallel plate type plasma of capacitive coupling type is more advantageous as compared to the inductive coupling type plasma with a large volume.
Further, for the purpose of improving controllability of Vdc and plasma density, there has been invented a method to introduce RFs with two different frequencies to parallel plate electrodes for independently controlling plasma density with a high frequency (100 MHz for example) RF and Vdc by a low frequency (3 MHz for example) RF (for example, refer to JP-A 2003-234331 (KOKAI)). In this case, in addition to a high frequency power supply and a high frequency matching device, there are provided a low frequency power supply and a low frequency matching device, thereby allowing superimposing of the aforementioned high frequency RF and low frequency RF with respect to the RF electrode.
In viewpoints of cleaning process and process stabilization, it is advantageous that the counter electrode is at the ground potential. When an RF is applied to the counter electrode, the counter electrode is scraped by Vdc generated on the counter electrode surface, which becomes a dust source or a source of unstableness for the process. Therefore, the two RFs are superimposed for the RF electrode on which the substrate is disposed.
Further, by pulsing of RF, there are attempted lowering of electron temperature (for example, refer to J. Appl. Phys. Vol. 86, No 9, pp 4813-4820 (1999)), suppressing of density of radicals disturbing the process (for example, fluorine radicals) (for example, refer to App. Phys. Lett., Vol. 63, No 15, pp. 2045-2046 (1993)), and improving selectivity of plasma etching (for example, a ratio of etching rate of silicon oxide/silicon) (for example, refer to J. Vac. Sci. Technol. A 13, pp 887-893 (1995)).
As described above, conventionally it has been attempted to suppress plasma damage due to lowering of electron temperatures or the like by pulsing of a high frequency RF (HF), or superimposedly applying a high frequency RF (HF) and a low frequency RF (LF) to control radical density.
Further, the present inventors are considering applying DC negative pulses and a high frequency RF (HF) superimposedly. In this technique, by superimposedly applying DC negative pulses, the band of positive ion energy becomes narrow and easily controllable to an energy band that is desirable for a process, thereby improving processing accuracy of plasma etching, suppressing plasma damage, and improving an embedding characteristic of plasma CVD. On the other hand, the radical density is controlled by pulsing of HF (RF), and it is expected that plasma damage due to reduction of electron temperature is suppressed.
By superimposedly applying the DC negative pulses and the pulsed high frequency RF (HF), for example F radical density decreases (isotropic etching decreases) in anisotropic etching of an oxide film by a CF4 gas, anisotropic etching by ion radicals of CF3+ or the like increases, and moreover the ion energy is controlled to a narrow band. Further, the radical density of CF2 or the like increases, which becomes a generation source of a side wall protective film (facilitation of anisotropy). With these radical species selecting effect and energy selecting effect, remarkable improvement in processing performance together with improvement in process controllability are realized.
However, as shown in FIGS. 7A and 7B which will be explained later (analytical results of simulating plasma density, electron temperature, and time variation of a process when a pulsed HF and DC negative pluses are applied superimposedly), the electron temperature lowers in a quite short time (5×10−6 seconds or shorter) as the high frequency power (HF) turns off, and generation of ions and electrons by electronic collision and ionization stops. In this what is called an afterglow state, when the DC negative pulses are applied, electrons and ions in the plasma are drawn out of the plasma, and the plasma becomes unstable and disappears. The disappearance of the plasma causes reduction of the process rate, device damage when reignition, and process unstabilization. Further, as shown in FIG. 9, when processing an insulator 100 such as an oxide film or a nitride film using the DC negative pulses, it is possible that a charge-up due to insufficiency of electrons occurs in a bottom portion 102 of a trench 101. When such a charge-up occurs, it then causes deterioration of processing shape due to ion drflection, etching stop, or damage to the device due to a charge voltage.
The present invention is made in view of the above-described conventional situation, and an object thereof is to provide a substrate plasma processing apparatus and a substrate plasma processing method which, in a plasma processing apparatus of what is called a parallel plate type, increase radical species density that is suitable for processing a substrate, and are capable of controlling the ion radical energy to an energy value and a narrow energy band which are suitable for processing a substrate to thereby perform fine processing, and further performing excellent embedding film forming.